This application is related to my co-pending application entitled "Radar Detection of Turbulence in Precipitation," Ser. No. 612,384, having the same filing date as this application.
This invention relates to the detection and measurement of velocity of radar targets in general and in particular to an analog technique for processing small Doppler shifts in frequencies of pulsed radar return signals from precipitation targets.
The detection and measurement of precipitation velocities has application in meteorology and aviation. In the field of meteorology, the study of these velocities and their associated effect on general weather conditions (such as cloud formation and other significant weather behavior) leads to a more accurate and complete forecast of weather patterns and aids in meteorological research. More importantly however, the study of atmospheric wind velocities is of general concern to the field of aviation in order that aircraft may be properly apprised of and avoid hazardous conditions. Reflected radar returns from rain, snow, hail, and ice provide a means of indicating the wind velocity in the general location of such precipitation. The pulsed radar returns from such meteorological targets are in the nature of a complex waveform having rapid fluctuations in amplitude and phase. The fluctuation rates are usually limited only by the pulse width or the bandwidth of the receiver. The complex nature of the signal represents the vector summation of the simultaneous returns from the plurality of scatterers in the radar pulse volume. The size, spatial position and velocity of the precipitation particles relative to the radar contributes to the nature of the received signal. This affects the instantaneous and average intensity and the instantaneous and mean frequencies as well as the statistical properties of these parameters.
Doppler radar provides a suitable technique for measuring relative motion between precipitation targets such as ice, snow, and rain, and the radar itself. The velocity of this precipitation is therefore an indirect indication of the movement of the winds. Moreover Doppler radar also provides a means for measuring the intensity of the precipitation because the radar is capable of measuring reflections which are proportional to the size and quantity of the rain, snow, ice, and hail returns. That is to say an increase in the quantity of the precipitation results in an increased return, and accordingly the magnitude of the precipitation is detected and indicated by the Doppler radar. Since the size and quantity of precipitation varies in location and direction throughout a particular storm, it becomes desirable to obtain an indication of storm intensity as a function of distance and location from the radar source. Doppler radar is also particularly well suited for detecting the velocity of radar returns moving toward or away from the radar source. A comparison of the movement of particles in adjacent spatial volumes of the radar beam gives an indication of the perturbations in the horizontal and vertical air movement. However, while crude velocity estimates have been obtained with conventional non-coherent radar, precise measurements have required the use of more complex and more expensive coherent radar.
There are significant drawbacks for the use of Doppler radar to provide storm intensity data, and velocity dependent characteristics. Specifically, methods presently exist to obtain precipitation velocity information from Doppler radar. All of these methods required a measurement to be made on a large nunber of samples of the return signal at each range (i.e. a specific distance from the radar source). Such techniques include the use of a simple boxcar circuitry which samples each return and holds the value until the next return and then employs a filter to provide a sinusoidal estimate of the mean Doppler frequency. To obtain this desired result, covering many ranges, large scale digital computers or complex data processors capable of storing many samples are employed and then are used to perform complex mathematical analysis of the values to obtain mean frequency indicative of the Doppler shift, the variance and the power spectral density of the return at that range. This must be done for each range and as a result the number of calculations is large.
Furthermore, because of the random nature of the return signal these methods require long sampling and computer times to provide usable data. All of the methods presently available are limited by the number of ranges at which they are capable of making measurements unless extremely large numbers of circuits are combined with an adequate computer to provide simultaneous storage and computation. This situation has proven to be unacceptable.
Other systems employ more simple circuitry and are sensitive to the spectral shape of the return signal. However, these systems are also limited by the sampling rate of the radar system to the measurement of frequencies less than the pulse repetition frequencies so that ambiguities and frequency folding of the data do not destroy the accuracy. The pulse repetition frequency limitations and the large number of samples which are required, combine to force the use of very slow and unacceptable scanning rates for the radar antenna so that the resultant data may very well be meaningless in a fast moving storm situation.
A significant reason for the existence of the problems in the art of singal processing is that the small Doppler shift, usually measured in Hertz, is superimposed upon a radar return having (after conversion) an intermediate frequency in the magaHertz range. In other words the meaningful variation of the return signal is so small when compared with the frequency of the radar return it becomes extremely difficult to remove it and to manipulate it so that useful information can be obtained.
Considering the above drawbacks I have devised a method of providing an instantaneous measurement of the small Doppler shift in the return pulsed radar return from precipitation targets.
It is therefore an object of this invention to provide a simple discrete system for the instantaneous and continuous measurement of small Doppler shifts.
Another object of this invention is to provide an analog technique for processing small Doppler shifts which are indicative of the movement of rain, snow, hail and their fall velocities and other velocity dependent characteristics.
And yet another object of this invention is to provide a technique for measuring small Doppler shifts in a high frequency signal such as a pulsed radar return.
A further object of this invention is to provide such a measurement while retaining a wide bandwidth and high sensitivity.
And yet a further object of this invention is to provide the measurement so that velocities which are expected from tornadoes and the like are readily measured without regard to the pulse repetition frequency of the radar.